Microelectrical mechanical systems (MEMS) are electro-mechanical structures typically sized on a millimeter scale or smaller. These structures are used in a wide variety of applications including for example, sensing, electrical and optical switching, and micron scale (or smaller) machinery, such as robotics and motors. Because of their small size, MEMS devices may be fabricated utilizing semiconductor production methods and other microfabrication techniques such as thin film processing and photolithography. Once fabricated, the MEMS structures are assembled to form MEMS devices. The fabrication and assembly of MEMS devices is typically called xe2x80x9cmicromachiningxe2x80x9d.
For optical switching, structures can be built which have a mirrored surface for reflecting a light beam from a sending input optical fiber to a separate receiving output fiber. By constructing a mirrored surface onto a movable structure, the mirror can be moved in to, or out of, the path of a beam of light. With more than one switch aligned in the beam path, the beam can be directed to one of several receiving fibers. These types of structures are generally known as xe2x80x9coptomechanical switchesxe2x80x9d.
Optomechanical switches can employ any of a variety of configurations. One configuration commonly used is a pop-up or flip-up mirror, as shown in FIG. 1. In a pop-up mirror switch 100, the mirror 120 is attached to a structure 130 which allows the mirror to be moved from a lowered position, where the mirror is held out of the beam of light B1 (as shown by the dashed lines), to a raised position, where the mirror has been rotated up into the beam B1 (as shown by solid lines). As can be seen, the mirror 120 rotates about a hinge 140 when being moved between the lowered and raised positions. The hinge 140 is positioned at the surface 110 of the switch 100. The mirror is raised by actuators 150. In its raised position, the mirror 120 is at an angle Al to the beam B1.
It has been found that pop-up mirrors like that shown in FIG. 1, generally have difficulties keeping the angle Al within the necessary tolerances. This is especially true the more the switch 100 is used. Maintaining the alignment of the mirror with the light beam is critical to the operation of any such mirror. Changing the mirror position, even a few tenths of a degree, can result in the reflected beam failing to be sufficiently aligned with the receiving fiber. That is, if the mirror is positioned at an angle which is outside its operating limits, the light beam will no longer be properly aligned with the receiving fiber, and as such, the reflected beam will not continue to the receiving fiber. This will cause not only the specific switch to fail, but will effectively make the entire switching device (i.e. an array of switches) useless.
Another problem with pop-up mirrors has been the inherent limited displacements provided by the comb (lateral) actuators they use. Sufficient displacement is critical as it is necessary to move the pop-up mirror completely into and out of the path of the light beam.
Another actuator which has been used with pop-up mirrors are scratch drive actuators. While these types of actuators can provide longer travel distances, they have large contact areas which are susceptible to stiction and charging. This causes repeatability problems in long term cycling.
To overcome the inherent problems of pop-up mirrors, switches have been constructed which position the mirror in a fixed upright position and move the mirror vertically into and out of the light beam. An example of such a switch is shown in FIG. 2. As can be seen, the switch 200 has a mirror 220, an actuator structure 230 and an actuator hinge 240. The switch 200 is positioned on surface 210. The mirror 220 is attached to the actuator structure 230 at a mirror hinge 260 and is supported by a latch 270. With the switch 200 in the lower position, the mirror 220 is held down near the surface 210 and in the light beam B2. Then, when the switch 200 is in its upward position, the mirror 220 is raised up out of the light beam B2.
In this configuration, the mirror 220 is kept in a position where the angle A2 of the mirror relative to the beam B2, is kept constant as the mirror 220 is moved from its raised position to its lowered position. This provides the advantage that, unlike with the pop-up switches, the angle A2 is not changed during the operation of the switch 200. This keeps angle A2 from departing from its allowable range during repetitive use of the switch. As such, the likelihood of failure of the switch due to misalignment of the mirror is greatly reduced.
As shown in FIG. 2, the mirror 220 is supported and held in place by the latch 270. During construction of the switch 200 the mirror 220 is raised from a horizontal position by rotating the mirror 220 about the hinge 260. The mirror 220 is retained in its upright or vertical position with latch 270. A typical configuration for latch 270 is shown in FIG. 3.
As set forth in FIGS. 2, 3a and b, the latch 270 has cut-outs 272 which are received in the catches 222 of the mirror 220, when the mirror is raised up to its operating position. As further shown in the enlarged view in FIG. 4, the engagement of catches 222 with cut-outs 272 causes the mirror 220 to become xe2x80x9clockedxe2x80x9d into a fixed vertical position relative to the actuator structure 230. The positioning of the cut-outs 272 along the length of the latch 270 will determine the angle of the mirror relative to the actuator structure 230 and consequently will determine the angle A2 of the mirror relative to the light beam B2.
Unfortunately, mirrors and latches with cut-outs, as shown in FIGS. 2-4, have had relatively large variations in the positioning of the mirror from switch to switch. These variations have resulted in corresponding variations in the angle of the mirror relative to the light beam. As a result, these switches have had a high occurrence of failures from improper alignment of the reflected light beams with the receiving fibers. The variations in the mirror positioning are due to the fact that there exists a relatively large range in the possible location of the contact points between the latch and the mirror structure. That is, the location where the mirror structure contacts the latch varies from switch to switch.
As shown FIG. 4, both the cut-out 222 of the mirror 220 and the cut-out 272 of the latch 270 have rounded corners 224 and 272, respectively. With rounded corner 224 contacting rounded corner 274, a large variation of the possible location of the contact point between the corners exists. As noted above, this positional range of the contact point produces a corresponding range in the possible positioning angle of the mirror 220.
The rounded corners 224 and 274 are produced when each device is etched during fabrication. When etching small corners, particularly small inside corners, of small thin film structures, rounded corners typically result.
As a result, the angle A2 of the mirror relative to the beam B2, can vary significantly, as shown in FIG. 2. Thus, there exists a corresponding large range in the positioning of the reflected beam B2xe2x80x2. This, in turn, causes a greater number of switches to fail since the reflected light beam B2xe2x80x2 is not properly aligned with the receiving optical fiber. With the reflected beam B2xe2x80x2 so misaligned, the receiving fiber cannot further transmit the light beam. That is, the misalignment of the reflected beams B2xe2x80x2 due to the rounded corners 224 and 274, causes failure of not just the particular misaligned switch, but effectively the failure of the entire optical switching device.
Therefore, a need exists for an apparatus which couples mircomachine structures together more precisely and which minimizes the range of possible positions between coupled structures.
In at least one embodiment, a thin film structure having a first structure, a second structure, and a latch mounted therebetween. The latch has a first end mounted to the first structure and a fastener connected to the second structure. The fastener has a fastener support surface and a fastener side surface, where the fastener support surface is in contact with the second structure. The fastener support surface and the fastener side surface are angled toward each other to define a fastener corner.
At the fastener corner is a fastener notch. The fastener notch functions to remove the curved corner which would otherwise be produced during the fabrication of the fastener corner. The removal of a curved corner at the fastener corner produces a substantially flat fastener support surface. The flat surface of the support surface improves the accuracy of the positioning of the connection of the latch to the mirror structure. This is because the contact point is no longer located on a curved surface which in prior devices can cause the location of the contact point to vary from connection to connection. Increasing the accuracy of the positioning of the contact point provides the distinct advantage that, the positioning of the mirror placed on the second structure, will be positioned with much greater precision. This, in turn, improves the alignment of the reflected light beam with the receiving optical fiber. The result is a great reduction in the likelihood of device failure due to beam misalignment. As such, a significant increase in production yield is achieved with the present invention.
For additional mirror positioning accuracy, the second structure can include a notched catch for receiving the latch. The catch has a catch support surface and a catch side surface. The catch support surface functions to receive the latch. The catch side surface and the catch support surface are angled to each other to define a catch corner. A catch notch is positioned at the catch corner, preferably on the catch support surface at the catch side surface.
As with the notch in the fastener, the notch of the catch functions to remove the structure at the catch corner, and as such, provides a substantially flat catch support surface. Since the catch support surface contacts the latch, the present invention""s more uniform support surface allows the second structure to be positioned with greater accuracy.
A preferred embodiment of the apparatus of the present invention is a MEMS optical switch having an actuator arm, two latches, a mirror hinge and a mirror structure.
The actuator arm is mounted on an actuator hinge so that it can be actuated up and down to move a mirror positioned on the mirror structure up and down, thus in and out of the beam of light. The latches are mounted to the actuator at a first end (opposite the actuator hinge) and each extend out to two fasteners on each latch. The fasteners are separated by a center portion. Each fastener has a fastener support surface and a fastener side surface. The fastener support surface functions to receive the mirror structure. The fastener support surface and the fastener side surface are angled to each other to define a fastener corner. The fastener corner has a fastener notch positioned on the fastener support surface at the fastener side surface. Because of the notch, the fastener support surface is substantially flat. A mirror hinge connects the actuator arm to the mirror structure.
The mirror structure is positioned at an angle to the actuator arm, preferably about 90 degrees. The mirror structure has the mirror on its surface and has two catches on either side of the mirror. The each catch receives a respective latch at the latches"" fasteners, securing the mirror structure in a fixed position relative to the actuator arm. Each catch has a catch support surface and a catch side surface. The catch support surface and the catch side surface are angled to one another forming a catch corner. Each catch corner has a catch notch positioned on the support surface at the side surface. The notch functions to remove the structure at the corners and in so doing makes the support surface flat. The latches are received on the flat catch support surface. The catch support surface is positioned between two catch side surfaces forming a trench. The trench receives the center portion of the latch. Forming the catch side surfaces are shoulders, one of which is located on each side of the catch support surface. The shoulders function to receive the flat fastener support surfaces.
In the preferred embodiments the apparatus is a polycrystalline silicon. The actuator arm and mirror structure are about 1.0 xcexcm and the latch is about 1.5 xcexcm thick.
The method of the present invention includes: providing the actuator arm and mirror structure, forming a sacrificial layer with a via to the actuator arm, forming a latch having a fastener with a notch and which is connected to the arm through the via, removing the sacrificial layer, moving the mirror structure relative to the actuator arm, and engaging the latch, at its fastener, with the mirror structure. For greater mirror positioning accuracy, the method can also include forming a notched catch.
In the preferred embodiments of the method, additional steps are included. Namely, the step of providing a first structure and a second structure includes forming a first structural layer and etching the first structural layer to define the first structure and the second structure. The step of forming a latch includes forming a second structural layer and etching the second structural layer to define the latch. Before forming the first structural layer it is preferred that the steps of providing a substrate, forming a poly 0 layer, etching the poly 0 layer, forming a lower oxide layer, and etching the oxide layer to form lower layer vias to the poly 0 layer.